Volume 524, December 2010
|Number of page(s)||16|
|Section||Interstellar and circumstellar matter|
|Published online||25 November 2010|
Physical properties of dense cores in Orion B9⋆
Finnish Centre for Astronomy with ESO (FINCA), University of
2 Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
Accepted: 7 September 2010
Aims. We aim to determine the physical and chemical properties of the starless and protostellar cores in Orion B9, which represents a relatively quiescent star-forming region in Orion B.
Methods. We observed the NH3 (J,K) = (1,1) and (2,2) inversion lines and the N2H+(3−2) rotational lines, with the Effelsberg 100-m and APEX telescopes, respectively, towards the submillimetre peak positions in Orion B9. These data are used in conjunction with our APEX/LABOCA 870 μm dust continuum data of the region.
Results. The gas kinetic temperature in the cores derived from the NH3 data is between ~9.4−13.9 K. The non-thermal velocity dispersion is subsonic in most of the cores. The non-thermal linewidth in protostellar cores appears to increase with increasing bolometric luminosity. The core masses, ~2−8 M⊙, are very likely drawn from the same parent distribution as the core masses in Orion B North. Based on the virial parameter analysis, starless cores in the region are likely to be gravitationally bound, and thus prestellar. Some of the cores have a lower radial velocity than the systemic velocity of the region, suggesting that they are members of the “low-velocity part” of Orion B. The observed core-separation distances deviate from the corresponding random-like model distributions. The distances between the nearest neighbours are comparable to the thermal Jeans length. The fractional abundances of NH3 and N2H+ in the cores are ~1.5−9.8 × 10-8 and ~0.2−5.9 × 10-10, respectively. The NH3 abundance appears to decrease with increasing H2 column and number densities. The NH3/N2H+ column density ratio is larger in starless cores than in cores with embedded protostars.
Conclusions. The core population in Orion B9 is comparable in physical properties to those in nearby low-mass star-forming regions. The Orion B9 cores also seem to resemble cores found in isolation rather than those associated with clusters. Moreover, because the cores may not be randomly distributed within the region (contrary to what was suggested in our Paper I), it is unclear whether the origin of cores could be explained by turbulent fragmentation. On the other hand, many of the core properties conform to the picture of dynamic core evolution. The Orion B9 region has probably been influenced by the feedback from the nearby Ori OB 1b group, and the fragmentation of the parental cloud into cores could be caused by gravitational instability.
Key words: stars: formation / ISM: clouds / ISM: individual objects: Orion B / ISM: molecules / radio lines: ISM
This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX) under programme 084.F-9312, and observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at Effelsberg. APEX is a collaboration between the MPIfR, the European Southern Observatory, and the Onsala Space Observatory.
© ESO, 2010
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